Hello!
I was measuring rendering indexes for some of my designed LED light panels with Konica Minolta Spectrophotometer CL-500A. For some panels, it was giving the result, exceed rendering index range, and for some light panel, it was giving the negative value of Ra, (for example -16,-8,-44).
I know it is normal for some light sources to get negative rendering index, for example low pressure sodium light drops the rendering index in to negative (-47). Literature proposes in negative rendering to round it up with 0 and it is said that it has no color rendering ability at all.
If that is true, then what will be the difference between Ra -8 and Ra -44, there should be some difference between this negative values too? And how could rendering index exceed its 0-100 range that I get from device reading?
Thanking,
Bipu
Literature proposes that, negative rendering index should be round up by 0, and it means the light source does not have any rendering ability at all. But still question remains, what will be the difference between Ra -44 and Ra -8 ? And why the device output shows exceed rendering index range (0-100), what does that mean?
Bipul
Since color rendering index is defined as
Ra=100-4.6 ΔEuvw (1)
where ΔEuvw is the average color difference over eight standard test color checkers, expressed in the Euclidean metric in CIEUVW.
From Eq.(1), negative Ra simply menas ΔEuvw larger than 22.7.
Therefore, negative Ra should happen. Rounding up Ra100. If you do not mind, I will attach article in Japanese, which includes many negative Rn (n=9,...15).
Shigeo
Thank you very much Mr. Shigeo, it really a big help that you describe the rendering staff. My Japanese level is in konichiwa -arigatou, when I will develop this skill, I will ask from you the article.
Color rendering index (CRI or Ra) was defined and scaled using the equation above provided by Shigeo Kubota so that a "warm white" fluorescent lamp (with halophosphors, not a modern "warm white" fluorescent lamp) would have a CRI or Ra value of 50. Since the Ra value is linearly scaled based on the amount of chromaticty deviation, it can produce negative values. So an Ra of 100 implies no chromaticity shifts relative to the reference blackbody/daylight source, an Ra of 0 implied shifts twice as big as an Ra of 50, and an Ra of -50 would imply shifts three times as big as an Ra of 50.
As it has been mentioned above, CRI Ra scores quantify the difference in the colour appearance of objects under a reference light source (blackbody and daylight) and test light source. A score of 100 means there is no difference between the reference and test light source. A lower value does not indicate that a light source has no colour rendering ability, but only a bigger difference between the reference and test source. Since the colour space (CIE 1964 UCS) used for colorimetric calculations is not perfectly uniform, scaling CRI Ra differences woud not be very indicative.
Negative values is one of CRI's well-known limitations. Updated colour rendering metrics (e.g. CQS, TM-30-15, CIE Rf) use an additional mathmatical operation to limit the resulting value between 0 and 100.
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